Maleic anhydride is a substantial commercial product made throughout the word for over fifty years. It is used alone or in combination with other materials mostly as a precursor for other products, including resins, pharmaceuticals and food additives.
Hundreds of articles and patents have been published related to the vanadium phosphorus oxides catalysts since Bergman et al, U.S. Pat. No. 3,293,268, taught the process of oxidizing saturated aliphatic hydrocarbons to produce maleic anhydride using such catalysts, often referred to as mixed oxides of vanadium and phosphorus. Bulk analysis of the active, mature catalyst shows the catalyst to be generally crystalline vanadyl pyrophosphate. However, as yet there are many factors not clearly understood that are important to the making of active, mature catalysts giving commercially acceptable productivities, yields and lives.
Numerous methods of making the vanadium-phosphorus oxide catalysts with and without promoters are disclosed and taught in the prior art. Generally, such catalysts are made by contacting suitable vanadium compounds under conditions which result in the vanadium being in the +4 valence, and reacted with the phosphorus to form a catalyst precursor consisting essentially of hydrated vanadyl hydrogen phosphate. The catalyst precursor is subsequently recovered by techniques well know in the art, such as drying, filtering and centrifuging, and treated physically and thermally by several conventional practices to form "calcined" mature catalysts.
Very few methods don't use calcination as an integral part of the process of production of an active catalyst. U.S. Pat. No. 4,317,778, for instance, describes a process where the final activation of the catalyst precursor is claimed to occur by introducing the catalyst precursor into water to form an aqueous slurry and by spray drying the slurry to form microspheroidal catalyst particles to be used in fluid bed reactors.
The methods used for the calcination of the catalyst precursor may be divided into two broad categories
1) calcination performed in equipment other than the reactor (external calcination) and PA1 2) calcination in the reactor tubes, under hydrocarbon and air, usually mild operating conditions (in-situ calcination). PA1 a) an initial heat-up stage in an atmosphere of air, steam and nitrogen, PA1 b) a rapid heat-up stage at a programmed heat-up rate in an air/steam atmosphere and PA1 c) a maintenance-finishing stage, using consecutively an oxygen containing and a non-oxidizing atmosphere. PA1 a) introducing a pentavalent vanadium-containing compound into an olefinic, oxygenated organic liquid medium; PA1 b) effecting reduction of at least a portion of said vanadium to a valence state of +4; PA1 c) adding a phosphorus-containing compound to said medium to form a catalyst precursor precipitate; PA1 d) recovering the catalyst precursor precipitate; PA1 e) drying the catalyst precursor precipitate; PA1 f) calcining the catalyst precursor precipitate. PA1 a) contacting a phosphorus compound and vanadium compound in an organic solvent under conditions which will provide a catalyst precursor having a phosphorus to vanadium atom ratio between about 0.9 to 1.2 and having more than 90 atom percent of the vanadium in the tetravalent state; PA1 b) recovering the precursor; PA1 c) drying the precursor, limiting the maximum temperature in an oxygen-containing atmosphere, to a value which will not allow any substantial oxidation of the residual organic materials; PA1 d) submitting the precursor, prior to calcination, to a chemical pretreatment by contacting with dry inert gas containing vapors of an aliphatic anhydride, having from 4 to 8 carbon atoms, preferably acetic anhydride, at a temperature not to exceed about 200.degree. C.; PA1 e) providing an atmosphere selected from the group consisting of air, steam, inert gases and mixtures thereof, and calcining the precursor, in said atmosphere, by raising the temperature, as measured in the precursor, above that attained in step (d) at a rate of less than 1.degree. C. per minute to a temperature greater than 350.degree. C., but no greater than 550.degree. C., and maintaining the temperature for a time effective in giving a vanadium oxidation state no greater than +4.5 and in completing the conversion to generate an active catalyst.
An external calcination method which results in a good, competitive catalyst has many advantages over the in-situ procedure. Firstly, productive capacity is lost, usually for weeks, during the in-situ calcination operating at below normal feed concentrations and throughput. Secondly, since the calcination procedure is a very sensitive operation which, if done improperly, results in inferior catalysts, the total reactor charge is put at risk in the in-situ calcination procedure, since the whole catalyst charge is calcined at the same time. The external calcination has the advantage of calcining the catalyst in smaller increments, resulting not only in lower risk of inferior catalyst charged to the commercial reactor, but allowing known procedures for measuring and controlling the quality of the catalyst. Better performance in yield, productivity and life results.
Prior art teaches procedures for both in-situ and external calcination. In both methods the ultimate form of the mature catalyst, in the bulk, is crystalline vanadyl pyrophosphate with various degrees of activity and selectivity for the production of maleic anhydride. Usually in the in-situ method the catalyst in the precursor form is charged to the reactor and brought up to reacting conditions using a feed of hydrocarbon and air. After several days or weeks of producing maleic anhydride at low rate, the precursor is converted to the active vanadyl pyrophosphate with the bulk of the vanadium very close to a valence of +4.
Generally, in the external calcination procedures, the prior art teaches that the catalyst be partially oxidized during the calcination. For reason not totally understood, partial oxidation of vanadium is required to make catalysts with high performance. Vanadium oxidation levels of above 4.0 and below 4.8 are considered favorable. The external calcination procedures described in prior art are varied, using batch and continuous thermal systems. Gaseous atmospheres are controlled in many cases. Gaseous atmospheres containing a combination or mixture of hydrocarbon and oxygen are usually not used, because of the difficulty in controlling the exothermal oxidation.
U.S. Pat. No. 5,137,860 teaches a process for conversion of vanadium-phosphorus catalyst precursors to active catalysts by subjecting the catalyst precursor to elevated temperatures in three stages:
U.S. Pat. No. 4,562,268 relates to a process for the production of maleic anhydride by oxidation of aliphatic hydrocarbons in the vapor phase using phosphorus-vanadium mixed oxide catalysts. The catalysts employed are normally prepared by introducing pentavalent vanadium compounds into an alcohol capable of reducing the vanadium and contacting the mixture with alcohol modifying agents. The patent discloses two basic modes of calcination.: (1) air calcination and (2) nitrogen/steam calcination. In the air calcination the catalyst precursors are subjected to heating in air, as in one embodiment, to 400.degree. C. over a two hours period, then holding at this temperature for six hours. In the nitrogen/steam calcination the catalyst precursors are first calcined in air, at a temperature in the range from 325.degree. C. to 350.degree. C. for six hours, followed by calcination in nitrogen and steam at a temperature in the range from 250.degree. C. to 600.degree. C. for from two to ten hours. The nitrogen/steam calcination is preferred.
U.S. Pat. No. 4,392,986 discloses a process for preparing vanadium-phosphorus catalysts by reaction in isobutanol followed by water washing of the precursors. The precursors, after drying at 120.degree. C. to 140.degree. C., are activated in the reactor oxidizing butane in air to maleic anhydride, typifying the in-situ calcination type.
U.S. Pat. No. 4,336,198 relates to vanadium-phosphorus catalysts modified with uranium, in which the precursors are supported on inert porous media such as alundum shapes. Calcining of the coated particles is disclosed as "heating from 200.degree. C. to 400.degree. C. at a rate of 5.degree./minute with heating at 400.degree. C. for one hour".
U.S. Pat. No. 4,317,777 teaches the production of maleic anhydride using vanadium-phosphorus catalysts by oxidizing a mixture which comprises a hydrocarbon of at least 4 linear carbon atoms and an oxygen containing gas, which compositions are above the flammable limit. All of the catalysts described in the 18 examples were calcined as typified by the description: "The catalyst was calcined in-situ by heating to 385.degree. C. at a rate of 9.degree./minute, whilst a 1.5% v/v n-butane/air mixture flowed through the bed at a GHSV of 1000 hr.sup.-1 ". After several hundred hours of operation the performances of the catalysts were evaluated.
U.S. Pat. No. 4,315,864 teaches a process for preparing catalysts useful in the production of dicarboxylic acid anhydrides comprising the steps of:
The calcination procedure was typified by the description: "The catalyst precursor was then tableted with 1% graphite being added, in a Buechler press to 11/8 inch. diameter. The tablets were then calcined in air from 150.degree. C. to 400.degree. C. at a rate of 5.degree. C. per minute, being held at 400.degree. C. for 1 hour".
These and many other references teach various methods of calcining the vanadium-phosphorus precursor to produce catalysts which in turn produce maleic anhydride with more or less efficiency. The prior art does not teach the benefit of a chemical pretreatment of the catalyst prior to its final activation by calcination, as described in the instant invention. Furthermore the prior art does not teach the benefit of combining such chemical pretreatment with a slow heat up rate above the temperature which will not substantially oxidize the residual organic materials arising from the organic media used, nor does the prior art teach that the slow heat up rate of preferred range of the instant invention, from about 150.degree. C. to about to about 550.degree. C., following the chemical pretreatment, produces catalysts improved in activity, productivity and yield. On the contrary, the procedures, in which rate of heating is mentioned, and/or programmed, teach increasing the temperature at 1.degree. C./minute or higher.
As the only apparent exception, U.S. Pat. No. 5,847,163 mentions a heat up rate of from 0.1.degree. C. per minute to about 10.degree. C. per minute. Such a wide range of heat up rate is not associated with any chemical pretreatment, but only to the transformation of the precursor into an active catalyst in a fluidized bed.
Furthermore, U.S. Pat. No. 5,867,163 does not show any benefit of operating the calcination at heat up rates of less than 1.degree. C. per minute, specifying, on the contrary the preferred heat up rate to be in the range from about 1.degree. C. per minute to about 4.degree. C. per minute (see column 4, lines 20-21).
All references cited by this specification are incorporated by reference in their entirety.